From avian and mammalian hosts, several type C viruses have been isolated which have the ability to induce malignant transformation. Such viruses have been shown to have the so-called "sarc" sequences which probably code for a protein responsible for the viral mediated cellular transformation. The purpose of this work is to identify the product of the "sarc" gene, translating in a "cell-free" system the RNAs extracted from the virus itself and from different virus-infected cells, and to identify the product of the translation with the immunoprecipitation with specific antisera and with the hybrid-arrested translation method (HART) using specific cDNA probes.